Flash Flooding and Flood in California, March 11 to 13, 2025

Area Forecast Discussion
National Weather Service San Diego CA
845 PM PDT Mon Mar 10 2025

.SYNOPSIS...
Cool, wet, and windy weather is expected for much of this week.
There will be two main periods of rain and mountain snow, the first
on Tuesday and the second on Wednesday night into Friday morning.
Scattered light showers may linger through the day Wednesday, but
some places have the potential to see mostly dry conditions.
Periods of gusty southwest to west winds are expected Tuesday,
with stronger and more widespread westerly winds expected
Wednesday evening through Thursday. A few showers may linger into
Saturday, followed by a brief period of fair weather late Saturday
into Sunday, then there is the potential for yet another system
sometime early next week.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

This evening, satellite imagery shows a large shield of mid and
high clouds spreading across the region from the west ahead of
the low pressure system bearing down on SoCal. A band of showers
ahead of the low is about 100 miles offshore and could reach the
San Diego and Orange County coastlines around 5 am...give or take.
Lightning is being detected in the band of convection closest to
the center of the upper low, about 390 miles west southwest of San
Diego.

From previous discussion...
This low is expected to bring areas of rain, mountain snow, and
gusty southwest to west winds on Tuesday. Guidance is showing a
weak atmospheric river reaching the coast Tuesday, with IVT of
250/300 kg/ms. High resolution model guidance is indicating
showers could reach the coastline early Tuesday morning. The cold
core of the upper level low will move over Southern California
Tuesday afternoon, bringing some instability and chances of
thunderstorms with it. Highest chances for thunderstorms are in
San Diego and Orange Counties. Rainfall rates will be 0.30 to
0.60" per hour with a 10% chance of rainfall rates exceeding 0.60"
per hour. Localized areas of flooding are possible where the
heavier showers develop. Current rainfall estimates have
increased for coastal areas, due to the potential of heavier
convective bands developing. Forecast rainfall for the coast is
0.40 to 0.60"; for the valleys 0.25 to 0.40; for the mountains
0.50 to 1"; and for the deserts less than 0.20". Periods of gusty
south winds are expected Tuesday, with peak gusts of 30 to 40 mph
for the mountains and deserts and up to 25 mph along the San Diego
coast. Snow levels will be 5500-6500 ft, with 3-5 inches of snow
possible.

Most of the shower activity should wrap up by Wednesday, but
scattered light rain or drizzle are still possible. The most likely
place for additional showers is the mountains. By Wednesday
afternoon, southwesterly winds will begin to strengthen ahead of
another approaching upper level trough, especially for the
mountains and deserts.

The passing trough late Wednesday into Thursday has more
potential for widespread rain with an atmospheric river with near
500 kg/ms of IVT making landfall Thursday morning. This system is
expected to be colder than the Tuesday system with 700 mb
temperatures dropping to around -10C, translating to snow levels
potentially as low as 3000-3500 ft by Thursday night/early Friday
morning. The cold nature of the storm also increases the chances
of thunderstorms across the area, leading to the potential of
locally heavy rain and flooding. High resolution guidance is
indicating rainfall rates of 0.50 to 0.75" per hour, locally up to
1" per hour where thunderstorms develop and in the mountains
below the snow level. National Blend of Models chances for
rainfall totals exceeding 1 inch are 70 to 90 percent for the
coast and valleys and less than 10 percent for the deserts.
Chances for exceeding 2 inches are 70 to 90 percent for the
mountains and 40 to 50 percent for portions of the valleys. There
is even a 30 to 50 percent chance of rainfall totals exceeding 4
inches of rain on the coastal slopes of the San Bernardino
mountains. For the deserts, there is a 30 to 40 percent chance of
rainfall totals exceeding 0.50 inches. Estimated snowfall totals
of 3-5 inches below 5000 ft, 6-12 inches possible above 6000 ft,
and 12- 24 inches above 7000 ft. 1-2 inches is possible at 4500
ft.

Widespread gusty winds are expected for Wednesday night into
Thursday. Winds will begin to strengthen over the mountains and
into the deserts Wednesday evening. By Thursday, widespread gusty
winds are expected from the coast to the deserts. Chances for wind
gusts to exceed 35 mph for the coast is 30 to 40 percent, with
lesser chances in the valleys. For the mountains and deserts, peak
wind gusts of 55 to 65 mph are likely.

For the weekend, there is a little more divergence between model
guidance. Some solutions suggest a more zonal flow with a chance of
some showers continuing going into Saturday, whereas others display
more ridging occurring over the weekend, which would result in drier
conditions and slightly warmer temperatures. Ensemble solutions
indicate a more unsettled pattern taking shape early next week as
yet another low pressure system could impact the region.

&&

.AVIATION...
110300Z...High clouds continue to move into the region from the
southwest this evening ahead of a weather system. Rainfall and
associated low clouds with bases 3000-5000ft MSL move in after 10Z,
with a 60-80% chance for widespread -RA/-SHRA and brief VIS
reductions to 4-6SM for coasts and valley sites after 12Z. Bouts of
heavier RA with even lower VIS and MVFR CIGs after 14Z.

-RA/-SHRA/-SN/-SHSN then move west into the mountains 13-16Z, with
obscurations for elevations at or above 3000-4000ft as clouds build
in. VIS down to 0SM where clouds intersect the mountains. Heavier
bouts of precipitation expected after 16Z in mountains with
accumulating snow possible above about 6000 ft MSL. -SHRA in deserts
as early as 16Z Tue, more likely after 20Z. Iso TS possible for all
areas from the mountains westwards 14Z Tue through 03Z Wed, highest
for coastal San Diego County. -SHRA/-SHSN tapering off after 03-04Z
Wed, though low clouds 2500-3500 ft MSL and terrain obscurations
will linger.

&&

.MARINE...
Southeast winds increase early Tuesday to 15-20 knots with gusts to
25 knots through the day, before decreasing in the early evening.
Seas up to 10 feet and isolated thunderstorms will be possible
Tuesday morning through the early evening. A Small Craft Advisory is
in effect and contains more information. Strong southwest winds
increase late Wednesday night, turning northwest and strengthening
further with gusts approaching gale-force Thursday afternoon. These
winds will produce hazardous boating conditions Thursday and Friday.
Thunderstorms will again be possible late Wednesday night through
Thursday evening.

&&

.BEACHES...
A series of west to northwest swells associated with a few storms
will arrive this week. The first swell Tuesday will lead to elevated
surf of 3 to 6 feet, with isolated sets up to 8 feet, generating a
high rip current risk and potentially dangerous swimming conditions.
Similar surf is likely Thursday into Friday. There is also a slight
chance of thunderstorms and lightning strikes Tuesday morning
through early evening, and again Wednesday night through Thursday
afternoon.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from Wednesday evening through Thursday afternoon
     for Orange County Coastal Areas-Orange County Inland Areas-
     Riverside County Mountains-San Bernardino County Mountains-
     San Bernardino and Riverside County Valleys-The Inland
     Empire-San Diego County Coastal Areas-San Diego County
     Mountains-San Diego County Valleys-Santa Ana Mountains and
     Foothills.

     Winter Weather Advisory from 6 AM to 10 PM PDT Tuesday for San
     Bernardino County Mountains.

     Winter Storm Watch from Wednesday afternoon through Friday
     morning for Riverside County Mountains-San Bernardino
     County Mountains.

     Winter Storm Watch from Thursday morning through Friday morning
     for San Diego County Mountains.

PZ...Small Craft Advisory from 4 AM to 6 PM PDT Tuesday for Coastal
     Waters from San Mateo Point to the Mexican Border and out
     to 30 nm-Waters from San Mateo Point to the Mexican Border
     Extending 30 to 60 nm out including San Clemente Island.

&&

PUBLIC...PG
AVIATION/MARINE/BEACHES...CSP